This work is motivated by the industrial process of air (or dry) atomization of slag, where a high-speed jet of air impinges onto a free-falling stream of molten slag and disintegrates it into small droplets. The droplets solidify to form slag powder particles which may be used in applications such as sand blasting, roof shingles, and asphalt. Since different applications require slag powders with different ranges of particle size, controlling the particle size is of essence. A mathematical model is presented for the atomization process, which relates the Sauter Mean Diameter (SMD or ) of molten slag droplets to the atomizer design and operating parameters. The model solves a set of non-linear governing equations, including conservation of mass, momentum, and energy, in the integral form using an iterative method. This universal model can be applied to any combination of gas-liquid, and thus the model is validated with existing experimental results for air atomization of a water stream. A strong agreement exists between the calculated and experimentally measured values of . It is also found that at a constant air flowrate, the liquid flowrate affects the of droplets and the opening angle of the spray; the former is the main output parameter, and the latter is an intermediate parameter.